Information storage and optical recording systems are of rapidly increasing importance in the modern society in view of the exponential rise in the number and complexity of the data which must be recorded and be retrievable so as to handle the increasing everyday business workload, and to assist efficiently enough in scientific development. Many optical information storage systems, including those based on silver halide emulsions and the like, have contributed significantly to this development, generally because of the high packing density combined with good resolution of information retrieve inherent to such systems. The advantage of READ ONLY MEMORY devices, based on photochemical generation of fluorescent compounds lies in the fact that storage of information in such systems is rapid and accurate, and there is no fixing required. Reading is also rapid, exceptionally sensitive and accurate and is not accompanied by degradation. At the same time there is another requirement the modern optical storage systems should comply with. The reading process should be non destructive for the memory material so as to enable repeating of reading thousands of times without detectable deterioration of the medium material. The reading process, can be carried out either by scanning point by point of the same layer, i.e. bit by bit of information, or by simultaneous reading from a 2-D layer carrying a medium material with the data recorded therein. In this manner a very fast access time to the stored information can be achieved. Reading of this type can be implemented by illumination of a 2-D layer by a fiber optics means. The intensity of illumination radiation should be sufficient for triggering fluorescence within the medium material accompanied by subsequent fluorescent photo-emission from all sites of the layer where the data have been stored.
Various methods and systems of recording of information based on generation of fluorescent compounds obtained from non-fluorescent precursor have been proposed.
The known methods include, for example, UV irradiation of bis-diarylchloromethyl-1,3,4-oxadiazoles as described by Singh in U.S. Pat. No. 3,869,363, herein incorporated for reference. There are known also many others systems as described by Zweig in the paper "Photochemical Generation of Stable Fluorescent Compounds", Pure and Applied Chemistry, vol. 33, pages 389-410 (1973), herein incorporated for reference.
The main common feature of the prior art optical memory systems is associated with the fact that reading in those systems can be implemented only by lasers having wavelength shorter than 500 nm. It can be appreciated that this requirement prevents employing of modem lasers with longer wavelength.
Furthermore, there exists another requirement to the optical memory systems. This requirement is associated with the fact that the intermediate photo-induced fluorescent product should be thermally and photo-chemically stable otherwise it can be destroyed by the reading process itself. Unfortunately the known in the art materials employed in optical memory systems are not sufficiently stable.